Rotor blade



H. H. PLATT ROTOR BLADE Filed March 14, 1956 IN V EN TOR.

Dec. 19, 1961 Il 'lll Tl e ,l --1 L, ILINIF .r .lL |||||P L w Q vm t 2 h\ m r2 rw VN United States Patenthhc/e Patented Dec. i9, 1961 3,013,614 RTR BLADE Haviland H. Platt, i9 E. 53rd St., New York, NX. Filed Man 16.1, 195o, Ser. No. 571,566 l Claim. (Cl. 17d- 159) My invention relates to rotor blades for use in heli copiers, convertiplanes and other types of rotating wing aircraft, and more particularly to rotor blades with metal acting surfaces.

Hitherto metal rotor blades, which are desirable on account of their greater durability and accuracy of form than blades having fabric or wooden coverings, have been made by a number ot methods and with various types of metal envelopes. All these have suffered hown ever from one or more of the following deficiencies:

(l) Lack of accuracy;

(2) High cost of construction;

(3) Dirliculty of assembly;

(4) Dihculty of service repairs;

(5) Lack of uniformity;

(6) Weight variation;

(7) Balance variation;

(8) Susceptibility to fatigue failure;

(9) Excessive weight;

(l0) Difficulty in applying to twisted blades;

ll) Difficulty in applying to tapered blades;

(l2) Diihculty in applying to twisted and tapered blades;

(13) Buckling of the envelope skin;

(lit) Deformation under load.

One object of my invention is to provide a rotor blade iving a metal envelope which is uniformly pretensioned to prevent the occurrence ot local compression with consequent buckling and also to prevent bulging under load due to the sldn being drawn away from the ribs.

Another object of my invention is the provision of an arrangement of parts of `a metal rotor blade in'such a manner that they may be made by inexpensive and accurate mass production methods such as stamping, rolling, extrusion, die-drawing and the like.

Another object of my invention is to provide a metal rotor blade which is capable of being assembled and disassembled without the need for elaborate tools or fixtures and in which damaged parts can be readily and inexpensively replaced either in the shop or in the lield.

Another object is that of providing metal blades of extreme accuracy of form, weight and balance, complete interchangeability being thus attained and iield adjustment of rotor tracking being made unnecessary.

Still another object is that of providing a design and method of construction which is adapted to the production oi tapered and twisted metal blades having an accuracy of form, weight and balance, an ease of construction and a facility of assembly comparable to those of an untwisted and untapered blade.

Still another object is that oi providing a method of adhesive bonding of the parts of metal rotor blades which eliminates the need for dies and lixtures in applying the pressure between parts, as customarily required for this type of bonding.

With the above and other objects in View, which will appear more fully from the following detailed description, my novel construction, in its form at present preferred by me, consists of:

(l) A thin walled tubular metal envelope with the portion which is to become the trailing edge preformed by bending, the entire envelope being in one piece or comprising a number of separate sections;

(2) Agplurality of envelope-supporting rib members contoured to the form vto be given the internal surface of the envelope, the rib members having locating projections or stubs;

(3) A tubular rib-supporting member perforated with holes adapted to receive the locating projections of the ribs;

(4) A preformed leading edge counterweight Vstrip supported by the ribs and reinforcing the leading edge of the envelope;

(5) A central key spar extending the length of the blade and acting as an expanding key to put compression in the ribs, through the locating stubs, and thus place the envelope skin in pretension;

(6)' A tip block attached to the outboard end of the central spar to close the end of the blade;

y(7) A root attachment member fixed preferably to both the spar and the rib support tube and formed at its inboard end for attachment to a rotor hub.

in the accompanying drawings, in which like reference characters indicate like parts, l have illustrated in a general way a form of construction embodying the above structural and functional characteristics, although it is to be understood that the various instrumentalities of which my invention consists can be variously arranged and organized and that my invention is not limited to the precise arrangement and organization herein described.

Thus, FiGURE l represents a plan View, partly bro lien away, of a rotor blade or" my novel construction.

FlSURE 2 represents a sectional view on line 2-2 of FIGURE l to an enlarged scale.

FGURE 3 shows a fragmentary section on line 3-3 of FGURE 2 illustrating one method of introducing7 pretension in the skin during assembly.

FiGURE 4, corresponding to FGURE 2, illustrates an alternate form of rotor blade construction utilizing a key spar of H-section in piaceY of the tubular spar shown in Prout-:E 2.

FiGURl- 5 shows a fragmentary section, corresponding to FiGUlE 2, illustrating an alternate method of tensioning the blade envelope.

The envelope 5 of the blade of my novel construction is formed of thin metal sheet suchl as aluminum alloy or stainless steel. The4 envelope 'S'is preformed into a closed tube by Welding, cementing or by ,any other suitable means, the circumferential dimensionof the tube being that desired for the'contour ofthe finished blade.

The acute angle of the trailing edge o is preferably next formed by bending over a form or by any other convenient means. While l prefer to form the tube over a cylindrical arbor, the trailing edge bendingirnay be performed before closing the tube, an arbor of non-circular section being then employed to support the sheet while it is being welded.

v The envelope 5 may be formed in one integral piece for the entire blade or, optionally, it may be made in two or more separate pieces.

The final contour, which may be of any suitable airfoil form, is imparted to the exible envelope 5 by forming or supporting ribs 7 and 8, ribs 7 determining the contour of the trailing portion of the blade and ribs S determining the contour of the leading portion. Ribs7 and 8, which may be punched from sheet metal or formed by any other method of precise reproduction, are located and retained on the locating tube 9 by means of stubs 1li which fit into slots in the locating tube 9.

Fittingl slidably in the locating tube 9 is the main spar tube il which exerts pressure on the stubs l0 of the ribs i and thus assuring a strong tension in the envelope .5

throughout. The ribs 7 and 8 may be reduced in weight by lightening holes l2 which may be of any suitabley shape.. The locating tube 9 may also be lightened by' re-` ducing its outside diameter over a part of each space between ribs, although for simplicity in illustration it is shown in FlGURE l with constant outside diameter.

The contact load of the envelope S on the ribs 7 at the trailing edge 6 is distributed along the blade by a strip 13 which is fitted into appropriate slots in ribs 7. Vlhile strip 13 is shown as a flat sheet piece of uniform section it may be lightened by cut-outs of suitable form and distribution. Alternately, it may be of V-section.

Distribution of tension load is effected at the leading edge of the blade by the metal strip 14. Strip 14 is formed to the contour of the desired interior surface of the leading edge of the envelope by rolling, punching or the like, the leading edges of ribs 8 being recessed to receive it. The leading edge strip performs three functions:

(l) Et establishes a continuous firm contour for the leading edge of the envelope 5 over the entire length of the etective blade airfoil.

(2) lt provides a rigid and strong reinforcement for the leading edge, which is in service subjected to the most severe pressure and impacts.

(3) It acts as a counterweight to bring the center of gravity of the blade into substantial coincidence with the axis of the spars 9 and 11 and also with the axis of the center of pressure of the airfoil, which customarily is located under normal iiight conditions at a point about 25% of the chord from the leading edge.

For these purposes the strip 14 is preferably formed of strong and rigid material, such as stainless steel of a thickness greater than that of the sheet metal of envelope 5. Alternately, a rod or tube of circular cross section may be added inside the forward portion of strip 14, the weight of which is adjusted accordingly, to provide additional rigidity and a more forward distribution of weight if required.

in forms of construction in which envelope 5 is closed by a welded or otherwise bonded seam, the seam is preferably so located that it is backed up by the nose strip 14. lf relatively soft material such as aluminum alloy is used for the envelope 5 a thin abrasion protective strip may be applied at the leading edge and suitably bonded to envelope 5.

The tip of the blade may be closed by a tip block 15 suitably formed by forging, casting, machining or the like, the trailing portion being internally recessed for bal ance purposes. A bored recess in the tip block 15 may be provided to iit over the end of spar 11 to which the block 15 may be attached by a llush pin 16 or by any other suitable attachment means. Preferably the tip block 15 is beveled or recessed to fit into the outer end of the envelope 5, thus assuring correct register of the block and support for the envelope.

Attachment of the blade to a rotor hub may be made by any suitable means, the arrangement of FIGURE l being shown and described for purposes of illustration only. ln this specific embodiment an attachment litting 17 is formed with a rod or plug extension 13 iitting into spar 11 and providing reinforcing against bending at the root portion of the blade. Fitting 17 is secured to spar 11 and locating tube 9 by the taper pins 19 passing through suitably rearned holes in all three members. Any of the other well known forms of root end attachment may be substituted for the one here described without departing from the essence of my invention.

For simplicity the blade is shown without a closure for the root end of the envelope. Any of the known arrangements for this purpose may be added either with or without structural support for the trailing edge. If the conditions of operation of a blade of my novel construction require trailing edge support at the root this can readily be provided by a diagonal brace bolted at the outer end to an extension of the trailing edge strip 13 and Secured at its inboard end to the attachment tting 17.

In the modification of my novel construction shown particularly in FIGURE 4 the locating tube 9 is of a generally rectangular' cross section, formed by extrusion or the like. The key spar 11 is of H-section, the anges engaging tne stubs 10 of the forming or supporting ribs 7 and which extend through suitably located holes in tube 9. This form of construction provides more rigid support for the ribs and consequently somewhat grea er accuracy of form to the blades than is attorded by the tubular central spar. lt is particularly applicable to large blades in which high pretension of the envelope is required.

"lflre nethod of assembly of a blade of my novel construction of the type shown particularly in FlGURE. 2, at present preferred by me, is illustrated in FiGURE 3. A tapered plug 26 is inserted in the outboard end of the key spar 11. The ribs 7 and 8 are assembled on the locating tube 9 and the leading and trailing edge strips 14 and 13 fitted in place. The envelope S is then put in place by s.iding it over the assembled framework, the tree projection of the rib stubs 10 into the locating tube 9 being sufficient to allow adequate clearance for ready insertion of the assembled frame into the envelope 5.

The outboard end of spar 11 with the plu" 2t) in place is next inserted in the inboard end of tube 9 and pushed in until the plug 20 contacts the stubs 1d of the most inboard ribs 7 and 8. A bolt rod 21 is passed through the interior of tube 11 and through a centra] hole in plug Ztl until the ilange 22, integral with bolt 21 or fixed to it by any suitable means, engages the inboard end of spar 11. The bolt 21, being of a length to extend beyond the outboard end of tube 9 when flange 22 is engaged with tube 11, is fitted with a thirnble 23 bearing on the outboard end of tube 9. A nut 24 threaded on the end of bolt 21 serves to exert pressure to draw tube 11 into place coaxially with locating tube 9. As nut 24 is turned to draw tube 11 into place the plug 2% successively engages the stubs 18 of the ribs 7 and 8, forcing them outward and thus applying tension to the skin of envelope 5. When the outboard end of tube 11 has been drawn all the way through tube 9 and extends beyond it the distance required for attachment of the tip block 15, nut 24 is backed oif, bolt 21 withdrawn, and thimble 23 and plug 2i) are removed. The envelope 5 is then fully tensioned and the blade is ready for the securing in place of tip block 15 and root end fitting 17.

Preferably the spar 11, the ribs 7 and 8, the leading edge strip 14 and the trailing edge strip 13 are proportioned with suiicient flexibility to provide enough combined deliection under load to take up minor inequalities in the dimensions of the envelope 5 and the parts bearing on it. Further adjustment of the pretension, locally or over the whole envelope, may be made by changing the length of stubs 10 so as to provide more or less en gagement with plug 2li as it is forced through spar 11. Since the blade can be readily disassembled by a reversal of the assembly process, the first assembly may be made a trial one. Loose areas of the envelope may be noted, the spar 11 and envelope 5 removed, and the relevant ribs 7 and 8 extracted. The stubs 10 on these ribs may then be extended by inserting pins, or otherwise, and the blade reassembled.

An alternate method of providing the tension in the envelope is illustrated in FIGURE 5. In this construction the forward ribs 8 are made somewhat thicker so as to accommodate countersunk-head set-screws 25, which bear on spar tube 11 either directly or through a plunger 2.6. The end of plunger 26 may be flared and contoured to tit spar 11, as shown, so as to reduce load concentration. With this arrangement of my invention the rib stubs 10 do not project into the interior of the locating tube 9, and the plug 20 and bolt 21 are not needed for assembling the blade. Instead, after assembly, the screws 25 are individually tightened, thus assuring uniform distribution of tension over the blade span. The holes in envelope 5 and leading edge strip 14, required for adjusting screws 25, may be plugged or covered in any suitable manner.

This alternate method of tensioning may also be applied to the form of construction illustrated in FIGURE 4. An alternate method of applying envelope 5 is particularly available with the frame construction of FIGURE 5. According to this method the sheet metal skin, after the bending of the trailing edge'angle 6, is formed directly over the collapsed frame, its ends secured and the frame expanded by means of screws 25.

For services in which disassembly for repairs is not considered important and in which the utmost rigidity and xity of parts is desired, the components may be bonded by any of the metal-bonding methods known in the art. This is readily achieved in a metal blade of my novel construction by precoating with bonding material before assembly all surfaces which are to be secured together. The pressure applied in assembly then provides the clamping action at all joints without the need of external clamping dies, as usually required. When thermosetting adhesives are used the entire blade may be heated to the required temperature. If metals of different thermal expansion rates are used in the blade, such as for example a steel skin with aluminum alloy ribs, the heating will cause an increase of pressure during the bonding operation.

For the construction with the H-spar, as shown in FIG- URE 4, the assembly tools are modified to accommodate the central web of the spar member 11 and the altered location of the contacts with stubs 10.

In either of the types of construction of FIGURE 2 or FIGURE 4 the taper may be formed on the end of the spar 11 itself rather than on a separate plug 20, the hole in the tip block 1S being then formed with a taper to match that on the end of spar 11.

In the construction of a at tapered blade the envelope 5 is formed over a conical instead of a cylindrical arbor (or over the assembled frame), the ribs 7 and 8 are given the requisite variation of dimensions along the spar and the leading edge strip is formed with the required taper. Otherwise the construction and the process of assembly are the same as for the blade of uniform section. If there is a taper of thickness as well as of plan form the spar 11 and locating tube 9 are formed with a suitable taper also. Some of the tensioning of the envelope 5 through ribs 7 and 8 may then be effected by the spar taper as the spar 11 is pulled into place.

I contemplate in some applications of my invention, particularly in the construction of tapered blades, the simultaneous use of the two methods of tensioning illustrated in FIGURES 3 and 5. Thus the inner spar 11 is drawn in as illustrated in FIGURE 3 thereby placing nitial tension in the envelope 5. Additional tension, individually adjusted, is then introduced by means of setscrews 25 in the leading portions of the ribs 8.

For the construction of twisted blades the basic form 6 with the tubular spar, as illustrated in FIGURES 1, 2 and 5, is particularly applicable. The locating holes in the tube 9 are then laid out and perforated along a helical path so that when ribs 7 and 8 are assembled to thelocating tube 9 the contour of the assembly has the required twisted form;

A very important benefit to be drived from my invention, it will be observed, is the maintaining by a blade constructed in accordance with its teaching of correct form under load due to the putting into its envelope during construction greater tension stress than any compressive stress to be encountered in normal ight, buckling and bulging being thus substantially avoided.

As an example of the light-weight possibilities of my novel construction, a blade of 12" chord with the spar :axis and the center of gravity at the one-quarter chord station and having a skin of .010" thick duralumin or of .004" thick stainless steel can be constructed with a speciiic weight of about 2.5 pounds per foot of length.

I am aware that the invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and I therefore desire the present embodiments to be considered in all respects as illustrative and not restrictive, reference being had to the appended claim rather than to the foregoing description to indicate the scope of the invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

In a rotor blade, an envelope, a locating tube extending longitudinally within said envelope, locating holes in said tube, forming ribs supporting said envelope and having locating stubs penetrating said locating holes, a leading edge member mounted on said ribs and supporting and reinforcing the leading edge of said envelope, and a key spar arranged to contact and press outward said locating stubs.

References Cited in the file of this patent UNITED STATES PATENTS 1,558,942 Stout Oct. 27, 1925 1,813,096 Stenner July 7, 1931 2,090,038 Goddard Aug. 17, 1937 2,342,025 Watter Feb. 15, 1944 2,391,514 Reid Dec. 25, 1945 2,410,056 Fresco Oct. 29, 1946 2,482,936 Renoux Sept. 27, 1949 2,514,525 Stulen July 11, 1950 2,577,336 Lampton Dec. 4, 1951 2,637,405 Stanley May 5, 1953 2,638,170 Prewitt May 12, 1953 FOREIGN PATENTS 623,322 Great Britain May 16, 1949 

